1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to an active device.
2. Description of Related Art
Taking the currently most popular liquid crystal display as an example, it mainly consists of a thin film transistor (TFT) array substrate, a color filter substrate, and a liquid crystal layer interposed between the TFT array substrate and the color filter substrate. In a typical TFT array substrate, amorphous silicon (a-Si) TFTs or low-temperature poly-silicon TFTs are usually used as switching elements for various sub-pixels. In recent years, studies have shown that an oxide semiconductor TFT has higher carrier mobility than the a-Si TFT, and the oxide semiconductor TFT is advantageous over the low-temperature poly-silicon TFT in large-area and low-cost fabrication. Therefore, the oxide semiconductor TFT has the potential to become a key element for the next-generation flat panel display. However, the oxide semiconductor TFT is very sensitive to oxygen molecules in the atmosphere and readily absorbs oxygen or moisture thus affecting electricity and stability of the entire device. Therefore, how to effectively prevent oxygen and moisture from entering into the oxide semiconductor TFT has become a very important subject.